The evolution of dimorphic sex chromosomes in the genus Coccinia (Cucurbitaceae)

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Few land plants have specialized male and female (dioecious) sporophytes. In spite of this limited overall success, unisexuality in plants has evolved many times, implying the independent evolution of various forms of sex determination, including XY sex determination, homomorphic sex chromosomes, and heteromorphic sex chromosomes. The latter are known from Cannabis, Humulus, Silene, Rumex, and Coccinia, the Cucurbitaceae that is the focus of this proposal. All c. 29 species of Coccinia are dioecious, but chromosomes have only been studied in C. grandis, in which the Y chromosome is about 2.5 times longer than the X. We propose to investigate two questions; what led to the great length of the Y chromosome and what was the pathway of karyotype evolution in Coccinia? The hypothesis being tested in regard to the first question is that an accumulation of repetitive elements has led to Y elongation, which will be tested by using representational difference analysis and chromosome walking, and in parallel a digestion with restriction enzymes. We shall address the second question by generating a phylogeny for Coccinia and reconstructing the evolution of the karyotypes of as many species as possible (current living collection: six species). We will also use flow cytometry to check for sex-specific C values, representing the total nuclear DNA content. This has worked for sexing dried leaves in Silene and may allow us to infer the presence of heterosomes in herbarium material of Coccinia.